Respiration consists of ventilation--the movement of air in (inspiration) and out (expiration) of the lungs--and gaseous exchange across the alveolar membranes in the lungs. Oxygen is extracted from the air and carbon dioxide removed from the blood. The eventual delivery of essential oxygen to the tissues and cells, and their use of the oxygen to break down 'fuel' molecules for production of energy and carbon dioxide, make up tissue and cellular respiration. The structures and organs involved in this and the complex neural arrangement that regulates the process all make up the respiratory system.
Regulation
A complex system of nerves in the brain stem runs the respiratory system, with various inputs from the cerebral cortex and peripheral structures. The respiratory center, governing inspiration and expiration, is housed in the medulla oblongata. It receives inputs from higher centers to generate a rhythmic firing in the inspiratory nerves. The expiratory nerves usually fire only during forced expiration such as sneezing, coughing and times of airway obstruction. These impulses are carried by the phrenic nerve to the diaphragm and the intercostal nerves to the external intercostals between the ribs. These are the muscles used in quiet breathing. Other muscles on the chest wall are recruited in times of distress or exercise.
The medulla is affected mainly by the level of carbon dioxide in the bloodstream, which translates to dissolved acidity. It's excited by increasing levels of carbon dioxide and slowed by reducing levels. It also receives nervous input from chemoreceptors in the carotid artery and the aorta. These react to the oxygen content of the blood and usually come into play only when the oxygen content is drastically reduced, as in higher altitudes or in chronic cases of obstructive lung disease.
Ventilation
Following the firing of the inspiratory center, the diaphragm--a sheath of muscle demarcating the thorax from the abdomen--contracts, pulling downward like a plunger on the thoracic cavity. The external intercostals also contract, pulling the ribs upward and outward. These two actions increase the volume of the thoracic cavity, expanding the lungs and pulling air through the nostrils, trachea, bronchi and bronchioles into the alveolar sacs. This is where gaseous exchange takes place. The end of the nerve impulse results in a recoil in the lungs, relaxation of the diaphragm and intercostals squeezing out air from the thoracic cavity.
Thus, inspiration is active while expiration is usually passive. Any obstruction of the tubules in the lungs or restriction of the recoil of the lungs will require active effort from the internal intercostals and accessory muscles on the chest wall to evacuate the lungs as seen in asthma and other chronic lung diseases.
Gaseous Exchange
Air drawn into the lungs contains oxygen at a higher pressure than in the blood, encouraging movement of oxygen into the capillaries surrounding the alveoli (the grape, cluster-like balloons at the end of the bronchioles). Once in the blood, the oxygen binds to hemoglobin in the red blood cells for oxyhemoglobin complexes, the form in which it travels to tissues and cells. This binding sets up a gradient that maximizes the flow of oxygen out of the alveoli and into the blood. Carbon dioxide arrives in the lungs at a higher pressure than atmospheric air, encouraging the movement of carbon dioxide out of the blood and into the lungs to be carried out by expiration.
Any reduction of the permeability of the alveolar membranes affects the movement of these gases, as in interstitial lung diseases such as emphysema. A drop in atmospheric pressure, such as occurring at high altitudes, reduces the amount of oxygen crossing into the blood. A drop in the hemoglobin level, as in anemia, also reduces the amount of oxygen crossing into the blood. Carbon monoxide forms stronger bonds with hemoglobin and keeps oxygen from being carried, starving tissues of much-needed oxygen. Fluid in the lungs, as seen in heart failure and other causes of pulmonary edema, also impedes the gaseous exchange.
Any reason for increased blood carbon dioxide or reduced oxygen trigger increased respiratory effort and obvious difficulty in extreme situations.
Tissue Respiration
The gradients are reversed at the tissue level, with oxygen flowing into the thirsty tissues for use in internal combustion of fuel molecules in the mitochondria of the cells. This combustion releases carbon dioxide that builds up and overflows into the blood, where the red blood cells convert much of it to bicarbonate. This increases the amount transported back to the lungs.
Conclusion
Like no other system, the respiratory system clearly demonstrates the interlacing relationship between organ systems in cooperation and regulatory inputs. The nervous system, circulatory system and hematological system are all involved in this system. It's much more than just breathing in and out.
References
- A complex system of nerves in the brain stem runs the respiratory system, with various inputs from the cerebral cortex and peripheral structures
- Following the firing of the inspiratory center, the diaphragm, a sheath of muscle demarcating the thorax from the abdomen, contracts, pulling downwards like a plunger of a syringe on the thoracic cavity.
- A drop in atmospheric pressure as at high altitudes will reduce the amount of oxygen crossing into the blood
